This invention deals generally with heat pipes and more specifically with hybrid heat pipes which have different structures for their evaporator and condenser sections.
Grooved aluminum Constant Conductance Heat Pipes (CCHPs) are the standard heat pipes used in spacecraft thermal control. The capillary grooves, which are typically formed by extrusion, allow long heat pipes that carry high power. On the other hand, the heat pipes have several limitations:
One is that the maximum evaporator heat flux is relatively low, on the order of 5-15 W/cm2. At higher heat fluxes, boiling in the evaporator grooves can disrupt the liquid return, causing the heat pipe to dry out.
Another limitation is the adverse elevation in gravity affected environments, the distance that the evaporator is elevated above the condenser. CCHPs can only operate with a small adverse elevation. They are typically tested on earth with a small adverse elevation of 0.1 inch against gravity to simulate operation in space. Straight and bent heat pipes can also operate in gravity aided mode with the condenser above the evaporator. For a bent heat pipe the evaporator can be non-level, but in this case the evaporator itself must have no more than a small adverse elevation, on the order of 0.1 inch from end to end, to allow liquid supply to the entire evaporator during startup. This requirement may not be practically satisfied for planetary landers and rovers that require a higher adverse elevation while navigating on tilted surfaces, or around rocks and holes.
Capillary grooves are the standard capillary structure used in spacecraft CCHPs, diodes, and Variable Conductance Heat Pipes. These grooves have a very high permeability, allowing very long heat pipes for operation in zero-g, typically several meters long. One of their flaws is that they are suitable only for space, or for gravity aided sections of a heat pipe. The reason is that the same large cross section dimension responsible for the high permeability results in low capillary pumping capability. In addition, axial grooved CCHPs also have a relatively low heat flux limitation.
Grooved aluminum and ammonia heat pipes are designed to work with a 0.10 inch adverse elevation in a 1-g (earth) environment. This allows them to be tested on earth prior to insertion in a spacecraft. However, they are very sensitive to adverse elevation. Increasing the adverse elevation by 0.010 inch will significantly decrease the maximum power that the heat pipe can carry. For heat pipes operating on Earth, the Moon, or Mars grooves can only be used in horizontal or gravity-aided portions of the heat pipe. Another wick with higher capillary pumping capability must be used for sections with adverse elevations.
Loop heat pipes are currently used in place of CCHPs for higher heat fluxes, or to overcome an adverse elevation. The disadvantage of loop heat pipes is that they are significantly more expensive to fabricate, and often are more difficult to start-up, sometimes requiring start-up heaters.
Problems have also been observed in the startup of vertically oriented grooved heat pipes in a gravity field where the evaporator is positioned below the condenser. In small diameter heat pipes, the fluid will accumulate in the evaporator as a liquid pool and may cause a higher thermal resistance at start up. The heat must transfer through the liquid pool, until sufficient power and superheat is applied to start boiling in the liquid. In some cases start up heaters have been used to apply a high heat flux over a small area to initiate boiling. Dual heaters are sometimes used for redundancy. These heaters require logic to initiate them, and add mass, which is undesirable in planetary exploration.
These problems can all be solved with a higher performance wick that has a smaller pore size and consequently a greater capillary pumping capability. The higher performance wick can also be more tolerant to higher heat flux, because the smaller pores are more resistant to vapor disrupting liquid flow. While it would theoretically be possible to use a higher performance porous wick throughout the heat pipe, this would significantly reduce the overall heat pipe power, since the permeability of a higher performance porous wick decreases faster than the pore size. An excessively low permeability may increase the liquid flow pressure drop to an unacceptable level, so that the heat pipe can only carry very low power.